I do train, coach, car and plane in fairly even amounts and they all have their advantages from time to time. It must be borne in mind that the main reason why coaches exist is because the train is seen as prohibitively expensive by some people, so arguably coaches would hardly exist if train fares weren't high. Having said that, if you added up all the seats on scheduled coaches versus the number of train passenges on competing routes then you can see that coaches are a very minor player, in England at least. It is different in Scotland where coaches are very competitive in both frequency and journey time.
Apart from cost, coaches can be better than trains in certain respects. Travelling at night can be more pleasant by coach as the lights are switched off whereas trains are illuminated very brightly. There is a fairly comprehensive network of overnight coaches whereas most overnight trains have ceased. Coaches mostly run a 7 day timetable, meaning they are particularly competitive early on Sunday morning when traffic is light. Train services don't usually start very early on Sunday and when they do start they are often slower than usual.